In the recovery of oil from oil-bearing subsurface formations, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the formation. Thus, a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subsurface formations. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into an oil-bearing formation. As the water moves through the formation, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
One difficulty often encountered in waterflooding operations is the relatively poor sweep efficiency of the aqueous flooding agent; that is, the injected aqueous agent tends to channel through certain portions of the formation as it travels from the injection system to the production system and to bypass other portions. Such poor sweep efficiency or macroscopic displacement efficiency may be due to a number of factors, such as differences in the mobilities of the injected aqueous agent and the displaced reservoir oil permeability variations within the formation which encourage preferential flow through some portions of the formation at the expense of other portions.
Various techniques have been proposed in order to improve the sweep efficiency of the injected aqueous agent and thus avoid premature breakthrough at one or more of the wells comprising the production system. The most widely used procedure involves the addition of thickening agents to the injected aqueous agent in order to increase the viscosity thereof and thus decrease its mobility to a value equal to or less than the mobility of the displaced oil, resulting in a "mobility ratio" of oil to water which is less than or equal to one. Many polymeric thickening agents have been proposed for use in such mobility control operations, see U.S. Pat. Nos. 3,085,063; 3,984,333; 3,969,592; 4,100,232; and 4,222,881.
Another procedure of recovering oil from subsurface oil-bearing formations is disclosed in U.S. Pat. No. 3,224,228 and involves alternately flooding the formation with gas and water. Gas is injected into a watered-out formation or the waterflooded portion of an existing waterflooding operation until a desired gas saturation exists in the reservoir. Water is then injected and the gas-water injection cycles repeated until the water-oil ratio becomes uneconomical. Other alternating gas-water injection methods are described in U.S. Pat. Nos. 3,525,395 and 3,529,396. The theory behind these alternating gas-water injection methods is that the gas would cause relative fluid mobility barriers in the permeable streaks such that the following water could flood out more of the oil zone. The problem with this theory had to do with the injected gas channeling so badly due to its low viscosity that the benefits to oil recovery were limited.
Applicant's U.S. Pat. No. 4,640,357 discloses a method for recovering oil from fractured or stratified subsurface formations wherein a first flooding agent having a high viscosity is injected into the formation to form a filtercake on the walls of the fractures or permeable streaks of the formation only in the near vicinity of the injection well. A second flooding agent having an inorganic thickener is injected deep into the formation to completely fill the thief zones provided by the fractures or permeable streaks of the formation between the injection and production wells. A first flushing agent is then injected to clear the fractures or permeable streaks of the formation only in the near vicinity of the injection well of the inorganic thickener. A second flushing agent having a breaker material is injected to remove the filtercake from the walls of the fractures or permeable streaks of the formation only the near vicinity of the injection well. A third flooding agent having a low viscosity is injected to flood the reservoir through the open pore spaces made available by the removal of the filtercake near the injection well. The third flooding agent preferentially floods through the previously unswept hydrocarbon-bearing formation matrix thereby improving sweep efficiency.
U.S. Pat. Nos. 4,098,337; 4,161,351 and 4,637,467 teach the use of polymers to plug or partially plug high permeability zones of viscous oil-containing subterranean formations. The polymers are injected into the formation and polymerized in-situ to partially or completely plug the highly permeable zones in the formation. U.S. Pat. No. 4,098,337 also discloses injecting aqueous solutions of polymer degrading materials such as hydrazine, sodium hypochlorite and like chemicals into the treated formation to restore some of the formation permeability.
The present invention is an improvement over previous processes in that it injects a gel solution at high pressure into the formation that deeply penetrates and plugs the highly permeable zones and fracture face of the formation followed by injecting a breaker material at low pressure that breaks the gel and displaces it from the fracture face and the highly permeable zones of the formation in the vicinity of the injection well thereby facilitating subsequent injection of a flooding agent to displace oil through the low permeable unswept zones of the formation.